The present invention relates to the field of liquid crystal display devices.
Liquid crystals are commonly used in display devices. In nematic displays, which constitute the preferred application of the present invention, a nematic liquid crystal is used which is achiral or chiralized, e.g. by adding a chiral dopant. The orientation and the anchoring of the liquid crystal close to the surfaces are defined by alignment layers or treatments applied to the substrates. In the absence of a field, this serves to impose a uniform or slightly twisted nematic texture.
Most devices that have been proposed and implemented in the past are monostable. In the absence of an electric field, the device implements only one texture. This texture corresponds to an absolute minimum for the total energy of the cell. Under a field, the texture is deformed continuously and its optical properties vary as a function of the applied voltage. When the field is interrupted, the nematic returns to its single monostable texture.
Another class of the nematic display is that of nematics that are bistable, multistable, or metastable. Under such circumstances, at least two distinct textures that are stable or metastable in the absence of a field can be implemented in the cell, using the same anchorings on the surfaces. The terms xe2x80x9cbistablexe2x80x9d or xe2x80x9cmetastablexe2x80x9d are generally used to designate two states having the same energy or energies that are very close, and that are capable of lasting substantially indefinitely in the absence of an external command. In contrast, the term xe2x80x9cmetastablexe2x80x9d is used for states having energy levels that are slightly different and that are liable to switch after a long relaxation time. Switching between the two states is implemented by applying suitable electrical signals. Once a state has been written, it remains stored in the absence of a field because of the bistable (or metastable) nature of the crystal. This memory of bistable displays is most attractive for numerous applications. Firstly it enables images to be refreshed at a slow rate which is very favorable for reducing energy consumption in portable appliances. Secondly, in fast applications (e.g. video) the memory makes a very high multiplexing rate possible, thus enabling video to be displayed in high resolution.
A typical example of a known bistable display [document 1] is shown diagrammatically in FIG. 1. In that case, one of the bistable textures (T0) is uniform (or, generally, lightly twisted), whereas the other (T360) presents an additional twist of xc2x1360xc2x0. The spontaneous cholesteric pitch p0 of the material is selected so that p0≈2xc2x7d (where d is the thickness of the liquid crystal layer) so as to equalize the energies of the two topologically equivalent states T0 and T360. A third texture T180, which is topologically different from the textures T0 and T360 is also possible using the same anchorings, and its energy is lower since it is better adapted to the spontaneous twisting of the material. However, in the absence of a field, T0 and T360 remain stable and do not transform into T180 because of topological constraints. Under a strong electric field, a fourth texture is achieved that is almost homeotropic, with the molecules being perpendicular to the substrates almost throughout, except in the vicinity of the plates. It is this texture that makes it possible to switch between the metastable textures T0 and T360. The particular final texture is selected under hydrodynamic control launched at the end of the control signal (backflow effect).
Another example of a known bistable display [document 2] is shown diagrammatically in FIG. 2. The two bistable textures T0 (uniform or lightly twisted) and T180 which differ by twist of xc2x1180xc2x0 are topologically incompatible. The spontaneous pitch p0 of the nematic is selected to be close to four times the thickness d of the cell, i.e. p0≈4xc2x7d so as to make the energies of T0 and T180 substantially equal. Without a field there does not exist any other state of lower energy: T0 and T180 are genuinely bistable. Under a strong field, an almost homeotropic texture (H) is obtained, with at least one of the anchorings on the substrates being broken: the molecules are normal to the plate in the vicinity of this surface. At the end of the control pulse, the cell is guided towards one or other of the bistable states depending on whether coupling between the movement of the molecules close to the two surfaces is elastic or hydrodynamic: elastic coupling returns towards the T0 state while hydrodynamic coupling returns towards the T180 state.
In order to enable the information displayed on the device to appear, it is necessary for the textures it implements to have different optical properties. Most devices operate with polarized light and use additional optical elements: polarizers, filters, compensating plates, etc. These elements and their orientations relative to the anchorings on the two surfaces are selected as a function of the configuration of the display so as to optimize pertinent optical performance: contrast, brightness, color, viewing angle, etc.
For monostable displays, optimization must apply to an entire continuum of states implemented under fields of various strengths, because these states are displayed throughout the duration of an image. A very large number of optical geometries have been proposed and implemented for various devices, taking account of the particular features of each of said displays. For each display, the configurations of additional elements are also adapted depending on whether they are used in transmission or in reflection.
The optics of the two above-mentioned types of bistable display are very different from that of monostable devices. Firstly, throughout most of the time an image lasts, only two textures exist in the cells of the display: textures corresponding to the two bistable states. The optimum configuration must enable maximum contrast between these two states, while minimizing transient optical effects during switching, due to passing quickly through intermediate states under a field. Furthermore, the main difference between the two bistable textures, an additional twist of 180xc2x0 or 360xc2x0 is not a parameter that is available for optimization: it is imposed by the physical mechanism used for achieving two bistable states. In addition, bistable switching requires a strong electric field (close to 10 volts per micrometer (V/xcexcm)). The liquid crystal layer must therefore be very fine (d≈2 xcexcm to 3 xcexcm) in order to enable control to be performed using reasonable voltages so optical optimization must take these requirements into account.
Until now, bistable devices have been considered above all in transmission mode, which is the mode in which they were originally proposed.
However, bistable memory is very useful in reflection mode: a bistable display operating in reflection can retain and display an image for a very long time without consuming any energy, neither for its own operation (it is bistable), nor for lighting purposes (it does not require an internal light source).
Recently, certain particular reflective configurations have been proposed for bistable devices having a twist difference of 360xc2x0 [documents 3, 4, and 5]. They use a single polarizer parallel to the nematic director on the front substrate. The state with little twist T0 has twist of 63.6xc2x0 [document 3] and of xe2x88x9236xc2x0 [document 4]. The contrast specified in those two cases is less than 10 in white light.
The configurations that have been proposed in the past for reflective bistable displays operate in xe2x80x9cnormalxe2x80x9d contrast, i.e. with a black state that is uniform or of small twist (T0) and with a white state that is highly twisted (T180 or T360). That configuration which is relatively easy to implement can theoretically achieve contrast of about 60 in white light. Unfortunately it is very sensitive to variations in the thickness d and in the twist angle xcex94xcfx86 of the T0 state, both of which are inevitable because of technological reasons.
The present invention now has the object of proposing a novel display device based on liquid crystals and which presents properties that are better than those of previously known devices.
In the context of the present invention, this object is achieved by a reflective bistable display device characterized by the fact that it comprises:
a) a liquid crystal material contained between two parallel substrates having electrodes on their facing inside surfaces to apply an electric field to said liquid crystal, at least the front substrate and the front electrode being optically transparent;
b) alignment layers or treatments on the electrodes to orient the liquid crystal and enable at least two alternate distinct stable or metastable textures to be implemented in the absence of a field, where one of the textures is either non-twisted, or else twisted at a total angle lying in the range xe2x88x9290xc2x0 to +90xc2x0, and the other possible texture presents additional twist to left or to right through an angle that is essentially an integer multiple of 180xc2x0;
c) the thickness d of the liquid crystal layer being selected in such a manner that the product dxc2x7xcex94n is close to xcex0/4, where xcex0 is the center wavelength of the working spectrum band of the display and xcex94n is the birefringence of the liquid crystal for said wavelength;
d) means designed to apply electrical signals to the liquid crystal enabling it to switch between said two distinct textures and to remain in one or other of them after the field has been removed;
e) a polarizer associated with the front face of the device, placed inside or outside the device;
f) a specular or diffusing reflective element placed at the rear face of the liquid crystal, inside or outside the device, enabling light to pass twice through the device and return towards an observer or towards additional optical elements; and
g) a compensator placed between the polarizer and the reflective element, the compensator presenting an optical delay dcxc2x7xcex94nc that is close to xcex0/4.
The reflective bistable display thus proposed by the present invention offers numerous advantages.
It makes use of contrast that is xe2x80x9cinvertedxe2x80x9d with a T0 state that is white and a high-twist T180 or T360 state that is black. Using a single polarizer and a compensating plate introducing an optical delay close to xcex/4, it makes it possible to achieve configurations providing contrast of 50 to 60 in white light. Without losing optical quality, optimization of the device also makes it possible to reduce cell thickness, thereby making switching faster and reducing the control voltages needed for switching. Because contrast is inverted, the optical quality of the device remains very good even when there are large variations in d and xcex94xcfx86.
According to other characteristics of the invention:
the liquid crystal material comprises a liquid crystal or a liquid crystal mixture in a nematic phase;
the liquid crystal material comprises a liquid crystal or a liquid crystal mixture in a cholesteric or nematic phase doped by a chiral substance to enable the energies of certain textures amongst textures that are stable or metastable to be brought close together or equalized;
the liquid crystal, the alignment layers, and the means for applying the field are selected to enable switching under a field to be performed by breaking anchoring or by propagating defects between two textures that are bistable or metastable in the absence of a field, with the difference between the total twist angles in said two textures being essentially close to 180xc2x0;
the liquid crystal, the alignment layers, and the means for applying the field are selected in such a manner as to enable switching under a field to be performed by breaking anchoring, by in-volume continuous distortion, or by propagating faults between two textures that are bistable or metastable in the absence of a field, and the difference in total twist angles with these two textures is essentially close to 360xc2x0;
the compensating plate is placed between the polarizer and the liquid crystal;
the compensating plate is placed between the liquid crystal and the reflective element;
the compensating plate introduces an optical delay xcex94I lying in the range 0.15xcex0 to 0.35xcex0, where xcex0 is the center wavelength of the working spectrum band;
the compensating plate is oriented at an angle lying in the range 35xc2x0 to 55xc2x0 relative to the polarizer;
the compensating plate is oriented at an angle close to 45xc2x0 relative to the polarizer;
the optical delay d.xcex94n of the liquid crystal layer lies in the range 0.15xcex0 to 0.35xcex0, and preferably in the range 0.20xcex0 to 0.32xcex0, where xcex0 is the center wavelength of the working spectrum band;
the polarizer is a linear or an elliptical polarizer;
at least one of the electrodes contains a plurality of different segments in order to enable a plurality of independent pixels to be implemented on the same substrates and in the same device;
that the independent pixels are provided with independent means for applying the field;
the independent pixels are organized in a multiplexed passive matrix;
the independent pixels are organized in a multiplexed active matrix;
the polarizer is oriented at an angle close to 45xc2x0 relative to the director of the liquid crystal on the front face of the device;
the twist angle of the texture in the low twist state, the additional twist xc2x1mxcfx80 in the second bistable state (where m is an integer), the orientation of the polarizer relative to the alignment of the liquid crystal on the front face, the thickness of the liquid crystal material placed between the two substrates, and the birefringence of the liquid crystal are optimized in such a manner as to obtain optimum optical performance, in particular in terms of contrast, brightness, and color;
the optical axis of the compensating plate is oriented at substantially 45xc2x0 relative to the polarizer;
the compensating plate introduces an optical delay lying in the range 100 nanometers (nm) to 180 nm;
the polarizer is combined with the compensating plate in the form of a single element to constitute an electrical polarizer; and
the thickness of the liquid crystal material is less than 6 xcexcm.